U.S. patent number 10,240,925 [Application Number 15/083,190] was granted by the patent office on 2019-03-26 for gradient force disk resonating gyroscope.
This patent grant is currently assigned to HRL Laboratories, LLC. The grantee listed for this patent is HRL Laboratories, LLC. Invention is credited to Richard J. Joyce, Jonathan Lake, Raviv Perahia, Logan D. Sorenson.
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United States Patent |
10,240,925 |
Perahia , et al. |
March 26, 2019 |
Gradient force disk resonating gyroscope
Abstract
A gyroscope includes a vibratory structure, and a control
mechanism including at least a first electrode, and at least a
second electrode adjacent the first electrode, wherein the
vibratory structure is separated from the control mechanism by a
gap, wherein to drive a vibration in the vibratory structure, the
control mechanism is configured to apply an alternating electrical
voltage between the first electrode and the second electrode, and
wherein to sense motion in the vibratory structure, the control
mechanism is configured to apply a direct current voltage bias
between the first electrode and the second electrode.
Inventors: |
Perahia; Raviv (Calabasas,
CA), Lake; Jonathan (Hidden Hills, CA), Joyce; Richard
J. (Thousand Oaks, CA), Sorenson; Logan D. (Calabasas,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
HRL Laboratories, LLC |
Malibu |
CA |
US |
|
|
Assignee: |
HRL Laboratories, LLC (Malibu,
CA)
|
Family
ID: |
65811630 |
Appl.
No.: |
15/083,190 |
Filed: |
March 28, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01C
19/5684 (20130101) |
Current International
Class: |
G01C
19/5684 (20120101); G01C 25/00 (20060101) |
Field of
Search: |
;73/504.13 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
US. Appl. No. 14/024,506, filed Sep. 11, 2013, Kirby et al. cited
by applicant .
U.S. Appl. No. 14/456,808, filed Aug. 11, 2014, Kirby et al. cited
by applicant.
|
Primary Examiner: Huls; Natalie
Assistant Examiner: Young; Monica S
Attorney, Agent or Firm: Ladas & Parry
Claims
What is claimed is:
1. A gyroscope comprising: a vibratory structure; a first electrode
separated from the vibratory structure by a gap; and a second
electrode directly adjacent the first electrode, the second
electrode separated from the vibratory structure by the gap;
wherein to drive a vibration in the vibratory structure an
alternating electrical voltage is applied between the first
electrode and the second electrode; wherein to sense motion in the
vibratory structure a first direct current voltage bias is applied
between the first electrode and the second electrode; and wherein
to tune a resonant frequency in the vibratory structure a second
direct current voltage bias is applied between the first electrode
and the second electrode.
2. The gyroscope of claim 1 wherein the vibratory structure
comprises a dielectric.
3. The gyroscope of claim 1 wherein the vibratory structure
comprises silica, fused silica, silicon, low expansion glass,
silicon nitride, diamond, silicon carbide, or sapphire.
4. The gyroscope of claim 1: wherein the gyroscope is a disk
resonating gyroscope; and wherein the vibratory structure comprises
a disk.
5. The gyroscope of claim 1: wherein the vibratory structure
comprises a disk, the disk comprising a solid disk, a disk having a
plurality of openings in the disk, or a disk having a plurality of
closed loops.
6. The gyroscope of claim 5 wherein the plurality of openings in
the disk comprise curved slots or circular openings.
7. The gyroscope of claim 1: wherein the vibratory structure
comprises a disk; and wherein the first electrode and the second
electrode are adjacent to a periphery of the disk.
8. The gyroscope of claim 7: wherein the disk has a thickness; and
the first electrode and the second electrode have a height that is
substantially the same as the thickness of the disk.
9. The gyroscope of claim 7 wherein the control mechanism further
comprises: at least a third electrode; and at least a fourth
electrode adjacent the third electrode; wherein the third electrode
and the fourth electrode are adjacent to one another and adjacent
to the periphery of the disk and are on a opposite side of the
periphery of the disk than the first electrode and the second
electrode; wherein to drive a vibration in the vibratory structure,
the control mechanism is configured to apply an alternating
electrical voltage between the third electrode and the fourth
electrode; and wherein to sense motion in the vibratory structure,
the control mechanism is configured to apply a direct current
voltage bias between the third electrode and the fourth
electrode.
10. The gyroscope of claim 1 further comprising: a substrate;
wherein the first electrode and the second electrode are on the
substrate and separated from the vibratory structure by the
gap.
11. The gyroscope of claim 10: wherein the first electrode and the
second electrode are below the vibratory structure.
12. The gyroscope of claim 11 further comprising: at least a third
electrode; and at least a fourth electrode directly adjacent the
third electrode; wherein the third electrode and the fourth
electrode are above the vibratory structure.
13. The gyroscope of claim 12: wherein to drive an out of plane
motion in the vibratory structure a direct current electrical
voltage is applied between the first and third electrodes, between
the first and fourth electrodes, between the second and third
electrodes, between the second and the fourth electrodes, or
between the first and second electrodes and the third and fourth
electrodes.
14. The gyroscope of claim 12: wherein to sense an out of plane
motion in the vibratory structure a direct current electrical
voltage is applied between the first and third electrodes, between
the first and fourth electrodes, between the second and third
electrodes, between the second and the fourth electrodes, or
between the first and second electrodes and the third and fourth
electrodes.
15. The gyroscope of claim 10: wherein the first electrode and the
second electrode are near an edge of the vibratory structure.
16. A method of providing a gyroscope comprising: providing a
vibratory structure; and providing a first electrode separated from
the vibratory structure by a gap; and providing a second electrode
directly adjacent the first electrode, the second electrode
separated from the vibratory structure by the gap; wherein to drive
a vibration in the vibratory structure an alternating electrical
voltage is applied between the first electrode and the second
electrode; and wherein to sense motion in the vibratory structure a
first direct current voltage bias is applied between the first
electrode and the second electrode; and wherein to tune a resonant
frequency in the vibratory structure a second direct current
voltage bias is applied between the first electrode and the second
electrode.
17. The method of claim 16 wherein the vibratory structure
comprises a dielectric or a disk.
18. The method of claim 16 wherein the vibratory structure
comprises silica, fused silica, silicon, low expansion glass,
silicon nitride, diamond, silicon carbide, or sapphire.
19. The method of claim 16: wherein the first electrode and the
second electrode are below the vibratory structure.
20. The method of claim 19 further comprising: at least a third
electrode; and at least a fourth electrode directly adjacent the
third electrode; wherein the third electrode and the fourth
electrode are above the vibratory structure.
21. The method of claim 20: wherein to sense an out of plane motion
in the vibratory structure a direct current electrical voltage is
applied between the first and third electrodes, between the first
and fourth electrodes, between the second and third electrodes,
between the second and the fourth electrodes, or between the first
and second electrodes and the third and fourth electrodes.
22. The method of claim 20: wherein to drive an out of plane motion
in the vibratory structure a direct current electrical voltage is
applied between the first and third electrodes, between the first
and fourth electrodes, between the second and third electrodes,
between the second and the fourth electrodes, or between the first
and second electrodes and the third and fourth electrodes.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to U.S. patent application Ser. No.
14/024,506, filed Sep. 11, 2013, and to U.S. patent application
Ser. No. 13/930,769, filed Jun. 28, 2013 which are incorporated
herein as though set forth in full.
STATEMENT REGARDING FEDERAL FUNDING
None
TECHNICAL FIELD
This disclosure relates to gyroscopes and in particular to disk
resonating gyroscopes (DRGs).
BACKGROUND
A navigation grade gyroscope has a bias of less than 0.01 deg/hr
and an angular random walk (ARW) of less than 0.001 deg/rt(hr). One
type of gyroscope is a micro-scale disk resonator gyroscope (DRG).
In the prior art this type of gyroscope does not meet the standards
for navigation grade performance.
The prior art DRG designs rely on the DRG being conductive either
by choosing a conductive structural material, such as doped Si, or
by coating a dielectric structural material, such as fused silica,
with a thin metallic layer. The control structure of the DRG for
drive, sense, and tuning is usually a set of electrodes placed
between the resonating rings of the DRG. In the case of a Si DRG,
the ultimate performance of the gyroscope is limited by the low
material quality factor (Q) of silicon, which has a Q less than 100
k. In the case of a fused silica DRG the performance has been
limited by the metal coatings, with Q dropping from Q.about.1e6 to
Q.about.200 k after a coating of only 10s of angstroms of metal.
Furthermore, the tight gaps between the electrodes and vibratory
structure make fabrication of a symmetric structure extremely
difficult due to limitations of deep reactive ion etching
(DRIE).
An example of the prior art is U.S. Pat. No. 7,040,163, which
issued May 9, 2006. As shown in FIGS. 1A and 1B, this prior art
relies on an internal electrode structure 108A and the electrical
conductivity of the resonating structure 100. Another example of
the prior art is U.S. Pat. No. 7,581,443, which issued Sep. 1,
2009, and as shown in FIG. 1C, this prior art critically depends on
electrode structures 104 and 106, which are also on the resonating
structure 100.
U.S. patent application Ser. No. 14/024,506, filed Sep. 11, 2013,
which is incorporated herein by reference, describes a touch-free
drive/sense mechanism for a small and light micro-shell, which is
further described in U.S. patent application Ser. No. 13/930,769,
filed Jun. 28, 2013, which is incorporated herein by reference. A
DRG structure, which may be 8 mm in diameter and 125 um thick is
significantly different in size and proportions than a micro-shell,
which may be a 1 mm diameter hollow cylinder that is 350 um tall
with a wall thickness of 2 um. Thus a DRG has 1000 times more mass
than such a microshell. Due to that difference in size and mass, it
is unexpected that a gradient force mechanism for drive and
particularly for sense for a DRG would be sufficient to achieve
navigation grade performance of such a large structure. For such a
greater mass both sufficient force to drive the DRG as well as
sufficient sensitivity are needed.
What is needed is a device and method to drive, sense, and tune a
DRG without any electrode structures or coatings, so that a high
quality factor can be achieved. The embodiments of the present
disclosure answer these and other needs.
SUMMARY
In a first embodiment disclosed herein, a gyroscope comprises a
vibratory structure, and a control mechanism comprising at least a
first electrode, and at least a second electrode adjacent the first
electrode, wherein the vibratory structure is separated from the
control mechanism by a gap, wherein to drive a vibration in the
vibratory structure, the control mechanism is configured to apply
an alternating electrical voltage between the first electrode and
the second electrode, and wherein to sense motion in the vibratory
structure, the control mechanism is configured to apply a direct
current voltage bias between the first electrode and the second
electrode
In another embodiment disclosed herein, a method of providing a
gyroscope comprises providing a vibratory structure, and providing
a control mechanism comprising at least a first electrode, and at
least a second electrode adjacent the first electrode, wherein the
vibratory structure is separated from the control mechanism by a
gap, wherein to drive a vibration in the vibratory structure, the
control mechanism is configured to apply an alternating electrical
voltage between the first electrode and the second electrode, and
wherein to sense motion in the vibratory structure, the control
mechanism is configured to apply a direct current voltage bias
between the first electrode and the second electrode.
These and other features and advantages will become further
apparent from the detailed description and accompanying figures
that follow. In the figures and description, numerals indicate the
various features, like numerals referring to like features
throughout both the drawings and the description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A, 1B, and 1C show disk resonating gyroscopes having
electrodes in accordance with the prior art;
FIG. 2A and FIG. 2B, which is a detail of FIG. 2A, show a periphery
drive, sense and tuning mechanism, and FIG. 2C and FIGS. 2D and 2E
show a substrate drive, sense and tuning mechanism in accordance
with the present disclosure;
FIGS. 3A and 3B show vibrating mechanical modes, in which the
vibrating structure is uniform and does not have special features
or regions that correspond to drive, sense, or tuning structure in
accordance with the present disclosure;
FIG. 4A shows a schematic of a control mechanism fabricated and
placed in proximity to a DRG vibratory structure, FIG. 4B shows a
finite element model of the electric potential on the outer ring of
a DRG, and FIG. 4C, which is a detail of FIG. 4B, shows an electric
potential exerted on the outer ring of the DRG by a set of
electrodes with alternating potential in accordance with the
present disclosure;
FIG. 5A shows force applied to the outer ring of the DRG by
electrodes as a function of electrode-vibratory structure distance,
FIG. 5B shows capacitance as a function of electrode-vibratory
structure gap, FIG. 5C shows capacitive sensitivity to
displacement, and FIG. 5D shows Allan deviation plots for
electrical noise in the gyroscope originating from amplifier noise
combined with capacitive sensitivity to motion in accordance with
the present disclosure;
FIG. 6A shows a drive and tuning mechanism, and FIG. 6B shows a
sense mechanism in accordance with the present disclosure;
FIG. 7 shows the top and bottom configuration of the control
mechanism in accordance with the present disclosure; and
FIGS. 8A, 8B, and 8C show different DRG geometries possible with
the gradient control structures in accordance with the present
disclosure.
DETAILED DESCRIPTION
In the following description, numerous specific details are set
forth to clearly describe various specific embodiments disclosed
herein. One skilled in the art, however, will understand that the
presently claimed invention may be practiced without all of the
specific details discussed below. In other instances, well known
features have not been described so as not to obscure the
invention.
The present disclosure describes a disk resonating gyroscope (DRG)
that is controlled using a gradient electric field mechanism
without direct contact with the vibratory structure. Such a control
mechanism allows the vibratory structure of a dielectric DRG to
achieve its ultimate quality factor and therefore ultimate
navigation grade performance. The control mechanism of the present
disclosure removes the need for a conductive layer to be directly
deposited on the vibratory structure which limits performance in
some prior art DRGs. Furthermore, the control mechanism of the
present disclosure eliminates the electrode structures typically
found between the rings of the DRG, significantly reducing
complexity of fabrication and therefore improving the ultimate
symmetry achievable.
The present disclosure describes how to achieve the ultimate
mechanical quality factor (Q) for a fused silica DRG. The present
disclosure applies to dielectrics in general; however, preferred
materials are fused silica, low expansion glasses, silicon nitride,
diamond, silicon carbide, and sapphire. The present disclosure
replaces the prior art direct electrostatic control mechanisms with
gradient mechanisms, and changes the overall design of the DRG.
As discussed above, prior art DRG designs rely on the DRG being
conductive either by choosing a conductive structural material,
such as doped Si, or by coating a dielectric structural material,
such as fused silica, with a thin metallic layer. The control
structure of the DRG for drive, sense, and tuning is usually a set
of electrodes placed between the resonating rings of the DRG. In
the case of a Si DRG, the ultimate performance of the gyroscope is
limited by the low material quality factor (Q) of silicon, which
has a Q<100 k. In the case a fused silica DRG the performance in
the prior art has been limited by the metal coatings, with Q
dropping from Q.about.1e6 to Q.about.200 k after coating of metal
with a thickness of only 10s of angstroms. Furthermore, the tight
gaps between the electrodes and vibratory structure make
fabrication of a symmetric structures in the prior art extremely
difficult due to limitations of deep reactive ion etching
(DRIE).
The use of electric gradient force control structures solves the
challenges with the fused silica DRG in two ways. First, the
gradient control mechanism allows driving, sensing, and tuning the
mechanical modes of a dielectric DRG without direct application of
electrodes, and second the electrodes typically found between the
DRG rings are no longer needed, significantly reducing DRIE etch
selectivity ratios needed to fabricate the structure.
Because the present disclosure allows for drive, sense, and tune
control of the resonating structure without any electrode
structures, the DRGs of the present disclosure not only benefit
from the high quality factor of the dielectric materials and easier
fabrication, but in addition DRG structures can also be modified as
needed for improved performance and ease of fabrication.
As discussed above, it is unexpected that a gradient force
mechanism for drive and particularly sense would be sufficient to
achieve navigation grade performance of a DRG with such a large
mass. As such the present disclosure solves a long standing
technical hurdle that has prevented fused silica gyroscopes from
become leading gyroscope devices.
Referring now to FIG. 2A, the gradient force gyroscope of the
present disclosure has two distinct structures namely, the
vibratory structure and the control structure or mechanism. The
vibratory structure, may be a disk 10 that is elevated from a
substrate 12. The vibratory structure may be anchored by anchor or
pedestal 14 in the center of the disk 10, as shown in FIG. 2C, to a
base 15 on the substrate 12, as shown in FIG. 2E. The disk 10 may
be a dielectric material, such as silica, silicon nitride, diamond,
silicon carbide, sapphire, and any other suitable material. The
vibratory structure has orthogonal wineglass mechanical modes that
are sensitive to the Coriolis force. For a fused silica disk 10
with a diameter of 8 mm, and a thickness of 125 um, and an anchor
or pedestal 14 with a diameter of 4 mm, the n=2 wineglass modes are
at approximate frequency f.about.12 kHz. The vibrating structure 10
may be uniform and does not need to have any special features or
regions that correspond to drive, sense, or tuning mechanisms.
The control structure 16 of the present disclosure allows for
driving and sensing motion of the disk 10. The control structure
may also be used to tune a resonant frequency of the vibratory
structure 10. The control structure 16 is not in contact with the
disk 10. A portion of vibratory portion of the DRG, such as disk
10, may or may not be electrically conductive; however, the
vibratory structure is preferably a dielectric with the region of
the disk 10 that interacts with the electrodes being a dielectric.
If the vibratory portion of the DRG is conductive, then it must be
coated with a dielectric.
In first configuration of the control structure 16, shown in FIGS.
2A and 2B, the control structure 16 is along the periphery of the
vibratory structure or the disk 10. FIG. 2B is a detail of a
portion of FIG. 2A and shows electrodes 18 along the periphery of
the disk 10; however the electrodes 18 are not in contact with the
disk 10. In another configuration of the control structure 16, as
shown in FIGS. 2C, 2D and 2E the control structure 16 is on the
substrate 12 underneath the vibratory gyroscope or disk 10. FIGS.
2D and 2E are details of portions of FIG. 2C and show electrodes 20
on the substrate 12 at the edge of the disk 10, or electrodes 20 on
the substrate 12 below the disk 10, respectively. In FIG. 2E, the
base 15 is shown, but portions of the disk 10 are not shown in
order that the electrodes 20 on the substrate 12 below the disk 10
are visible. In both FIGS. 2D and 2E, the electrodes 20 are not in
contact with the disk 10.
As will be described further below, the control mechanism can be
either below, above, or both below and above the vibratory
structure 10. The control structure shown in FIGS. 2A and 2B may be
referred to as a periphery control mechanism. The control structure
shown in FIGS. 2C, 2D, and 2E may be referred to as an above/below
control mechanism.
FIGS. 3A and 3B show the n=2 wineglass mechanical modes having a
resonant frequency of f.about.12 kHz for a disk 10 of fused silica
with a diameter of 8 mm, a thickness of 125 um, and an anchor or
pedestal 14 diameter of 4 mm. The vibrating structure or disk 10 is
uniform and does not have special features or regions that
correspond to drive, sense, or tuning structures.
The following technical discussion demonstrates that both
peripheral or above/below control mechanisms can achieve navigation
grade performance, which requires that the control mechanism have
enough force to drive the vibratory structure into oscillations and
is able to sense motion with bias stability of less than 0.01
deg/hr, which is navigation grade. As an example in the following,
1 um of motion of the vibratory structure is used. These
calculations are meant for illustration only and are not limiting
on the scope of the present disclosure as to the specific
dimensions used in the example. Also, there is no frequency limit
to the efficacy of either control mechanisms which may operate over
a range from single Hz mechanical and electrical frequencies to GHz
frequencies.
FIG. 4A shows a periphery drive, sense, and tuning control
mechanism in accordance with the present disclosure. A set of
conducting electrodes 18 are located near the periphery of the
vibratory structure or disk 10. The gap between the electrodes 18
and the disk 10 can be as small as 10s of nanometers and as large
as 10s of microns depending on the resonant frequency. The same
type of ranges may apply to the thickness, width, and spacing of
the electrodes 18. The thickness or height of the electrodes 18 may
be matched to the thickness of the vibratory structure 10 to
maximize the performance of the control mechanism. Finite element
simulations (FEMs) have been used to test the DRG
configurations.
In an example FEM, the electrodes 18 were set to be 35 um wide with
a distance between electrodes 18 set to be 4 um. The thickness of
the electrodes 18 was chosen to be 125 um to match the thickness
for the fused silica disks 10 shown in FIGS. 3A and 3B. To mimic a
DRG vibratory structure a ring of width 25 um was used as shown in
FIG. 4B. The concept can be fully demonstrated by simulating only
1/16.sup.th of the structure.
To drive the vibratory structure 10 an alternating current (AC)
voltage is applied to alternating adjacent electrodes 18, so that
adjacent electrodes 18 have potentials that alternate in potential.
Therefore if a particular electrode 30, as shown in FIG. 4C, has a
potential at a particular time, then an adjacent electrodes 32 may
have an opposite potential at that particular time. So if
electrodes 30 have a positive potential, then the electrodes 32 may
have a relatively negative potential. However, it is not necessary
that the electrodes 30 have a positive potential and the electrodes
32 have a negative potential. What is required is that there is a
difference in potential or voltage between the electrodes 30 and
the electrodes 32. For example, if electrode 30 is at +5V then
electrode 32 can be any voltage less than +5V as long as there is a
difference.
As shown in FIG. 4C the electrodes 30 alternate with the electrodes
32. The result is that an alternating force is applied to the
vibratory structure 10. A maximum amplitude of the vibration of
disk 10 is achieved at the resonant frequency of the disk 10;
however, it may be best for stability to slightly detune from the
resonant frequency. To sense motion, a direct current (DC) bias is
applied to alternating sets of electrodes 30 and 32. So, for
example, electrodes 30 may be biased to a DC voltage of 10 volts
and the electrodes 32 may be biased to zero voltage. Any change in
capacitance due to motion of the disk 10 results in a change of
charge state at the electrodes 30 and 32, which causes a current,
which may be measured using a trans-impedance amplifier (TIA).
Finally to tune the resonant frequency of the vibratory structure
10, a DC voltage bias may either be applied directly to the
electrodes 30 and 32, or a bias from the average value of the AC
may be used.
In the example shown in FIG. 4C, a potential of 10V is applied to
alternating set of electrodes 18, such as electrodes 30 with the
electrodes 32 having 0V, then the electrodes 30 are set to 0V and
the electrodes 32 set to 10V, and this repeats. The value of 10V is
used for simplicity; however, other alternating voltages may be
used. The key is for there to be a potential difference between
alternating electrodes 30 and 32 to generate a fringe field. Larger
voltages (>100V) can be used as long as the field intensity
between the electrodes 30 and 32 does not reach levels
corresponding to dielectric breakdown between the electrodes.
In this example case, the gap between the electrodes 30 and 32 and
the vibratory structure or disk 10 is varied from 500 nm to 5 um.
The capacitance and radial force is then calculated using FEM
simulations versus the gap between the electrodes 30 and 32 and the
vibratory structure 10. The results are shown in FIGS. 5A, 5B, 5C
and 5D.
As shown in FIG. 5A, forces greater than 100 nN can easily be
achieved with gaps as large as 5 um. For reference, the force
needed to drive a DRG into an n=2 oscillation mode with an
amplitude of 1 um on resonance, for a Q.about.1e6 disk 10, is as
low as 0.1 nano Newtons (nN). That force scales linearly with Q, so
even with a lower quality factor of Q.about.100,000 a force of 1 nN
would be needed, which is still fairly small. Clearly there is
sufficient force to drive the vibratory structure or disk 10 and
the drive force is strong enough to be placed further than 5 um
away from the vibratory structure 10.
In the case that the electrode set 30 and 32 is used as a sense
mechanism, FIG. 5B shows the corresponding change in capacitance of
the electrode set 30 and 32 as a function of the gap between the
electrodes 30 and 32 and the vibratory structure 10. The derivative
of that curve, shown in FIG. 5C, corresponds to the sensitivity of
capacitance to a change in the gap. Sensitivity varies from -5 to
-35 aF/nm. Using a noise model from a commercial trans-impedance
amplifier (TIA), the electronic noise contribution to gyroscope
bias and angular random walk (ARW) can be calculated. The noise of
the TIA, which includes white and pink electrical noise, appears as
a false perceived rotation and scales with the capacitance
sensitivity. Allan deviation (ADEV) plots are shown in FIG. 5D and
show calculated electrical noise for sensitivities of 4 aF/nm, 5
aF/nm, and 6 aF/nm using a Linear Technologies LT1169 TIA. For this
calculation an amplifier bandwidth is set to 40 kHz. The mechanical
mode used is at frequency f of 12 kHz, for a mass of 852 ug, a
Bryan's factor of 0.3, and a Q of 1e6. The ADEV plot is divided
into three regions by dashed black lines. The three regions
correspond to rate grade, tactical grade, and navigation grade,
where the lowest region 35 is navigation grade. It can be seen from
the ADEV plots of the three sensitivities that achieving
sensitivity >5 aF/nm would allow navigation grade performance.
More precisely, the ultimate performance of the gyroscope is not
limited by the control mechanism for drive, sense, and tuning. It
can be shown that for this mechanical structure the mechanical
noise contribution is also at the navigation level and therefore
the full integrated device would yield <0.01 degree bias
stability. The structure thus far simulated covers only 1/16.sup.th
or the diameter of the structure. A factor of two times in
performance can easily be gained by sensing on opposite sides of
the vibratory structure 10. That, combined with optimization of the
electrode spacing and width, allows for less than a 0.001 deg/hr
bias and a correspondingly reduced ARW.
A concept analogous to the one developed for the periphery control
mechanism can be developed for a configuration where the electrodes
18 are placed on the substrate 12 below the vibratory structure 10,
as shown in FIGS. 2D and 2E. As before, a key feature is that the
electrodes 18 come in pairs of alternating potential and run
perpendicular to the direction of motion so that they will either
generate a force or sense motion.
FIGS. 6A and 6B show a control mechanism for drive/tune and sense,
respectively. As shown in FIG. 6A, a pair of electrodes 18 of width
(Ew), separated by a length (Es) are placed on the substrate 12 at
a height (H) below a vibratory structure 10. If viewed from above,
the dielectric beam 10 partially overlaps with the electrodes by a
length (L). In this configuration, the alternating voltages on
nearest neighboring electrodes 18 exert a force pulling the
vibratory structure along the length of the electrodes 18, shown as
Fr, which is a force is in the radial direction of the DRG, as well
as pull the vibratory structure down, as shown by arrow Fz. As has
been shown via simulations, the proportion of radial to vertical
forces can be engineered so Fr>Fz. Furthermore, the force in an
operating device may be applied in a sinusoidal fashion at the
resonant frequency of the vibratory structure. Motion in the
preferential radial direction is amplified by the quality factor,
which is Q.about.1e6 for SiO.sub.2. A direct current (DC) force in
the <z> direction, which is out of plane, adds an out of
plane control over the vibratory structure 10. An often cited
concern with flat vibratory structures 10 is their susceptibility
to out plane motion due to acceleration. A force in the <z>
direction can be used in a control loop to mitigate this out of
plane vibration. The converse is also a potential implementation,
because out of plane vibration can now be sensed, making the
structure both an out of plane accelerometer as well as an in plane
gyroscope.
As shown in FIG. 6B the control structure can also be used to sense
the motion of the vibratory structure 10. When a DC voltage is
applied across nearest neighbor electrodes 18, motion in the beam
10 translates into a change in capacitance. Using a current to
voltage conversion trans-impedance-amplifier (TIA), the capacitance
can be sensed.
The control mechanisms of FIGS. 6A and 6B can be either below the
vibratory structure 10, or above the vibratory structure 10, or
both above and below the vibratory structure 10. FIG. 7 shows a
control mechanism above and below the vibratory structure 10. A cap
40 is put over the vibratory structure 10 and has electrodes 42 and
44 with alternating voltages on nearest neighboring electrodes.
Electrodes 46 and 48 are on substrate 12 below the vibratory
structure 10. Doubling up the control structure on both above and
below doubles both the available force and sensitivity to motion. A
differential measurement of out of plane motion can now be
implemented to rebalance forces in both up and down directions. To
drive an out of plane motion in the vibratory structure 10, the
control mechanism may be configured to apply an direct current
electrical voltage between electrodes 42 and 46, between electrodes
42 and 48, between electrodes 44 and 46, between electrodes 44 and
48, or between electrodes 42, 44 and electrodes 46, 48. To sense an
out of plane motion in the vibratory structure, the control
mechanism may be configured to apply an direct current electrical
voltage between electrodes 42 and 46, between electrodes 42 and 48,
between electrodes 44 and 46, between electrodes 44 and 48, or
between electrodes 42, 44 and electrodes 46, 48.
As in the case for the peripheral control mechanism, one can
demonstrate performance of the control mechanisms of FIGS. 6A, 6B
and 7 via finite element model (FEM) simulations. A single
dielectric member may be simulated with two electrodes. The DRG
member for the simulation may be 200 um in length, 30 um thick, and
20 um in width. The two electrodes may be 30 um in length
(E.sub.L), 5 um wide (E.sub.w), and spaced by 4 um (E.sub.s). This
configuration is for illustrative purposes and does not limit the
present disclosure. The electrode widths and spacings can be varied
within fabrication tolerances and can be as small as 100s of nm to
as large as 10s of microns. The vertical spacing is varied in the
simulation between 1 and 5 um. Depending the size of the structures
the vertical spacing or gap between the DRG member and the
electrodes can be smaller (100s of nm) or larger (5-25 um). As
described above, an alternating 10V potential may be applied across
the electrodes.
To get a sense for total force and total sensitivity one must
calculate the values for a 1/16.sup.th of the DRG as described
above. To do so, an effective length is calculated for a DRG
structure with 22 rings between R=4 mm and R=2 mm. The motion of
the DRG is assumed to be linear, with 0 motion at R=2 mm and
maximum motion at R=4 mm. The total force is again significantly
larger than needed for driving the structure. The sensitivity
varies between .about.3 aF/nm and 23 aF/nm.
The same amplifier noise calculation can be done as before to
calculate the electronic Allan deviation (ADEV) limit of the full
structure. For gaps of 1 um, 2 um, and 3 um the ADEV is well into
navigation grade performance. Taking into account that there may be
opposing 1/16ths of the vibrating structure 10, as well as top 42
and bottom 18 electrodes, the noise floor due to the transduction
mechanism and the amplifier approaches 0.0001 deg/hr.
Now that it has been shown that a gradient force control mechanism
can be applied to a DRG and thus eliminate the need to have
integrated electrodes, the constraints on the shape of the DRG may
be significantly relaxed. The shape of the vibratory structure can
be chosen to optimize vibration immunity by increasing frequency,
reducing DRIE etching complexity by making larger features, and
even opening the door for a wet etched structure. For example,
FIGS. 8A, 8B, and 8C show different possible configurations for a
vibratory structure 10. FIG. 8A shows a solid disk. Another
configuration is a disk having a number of openings in the disk
with each opening having a shape such as a curved slot, as shown in
FIG. 2B or a circular opening, as shown in FIG. 8B. Yet another
configuration is a disk having a number of closed loops, as shown
in FIG. 8C.
Having now described the invention in accordance with the
requirements of the patent statutes, those skilled in this art will
understand how to make changes and modifications to the present
invention to meet their specific requirements or conditions. Such
changes and modifications may be made without departing from the
scope and spirit of the invention as disclosed herein.
The foregoing Detailed Description of exemplary and preferred
embodiments is presented for purposes of illustration and
disclosure in accordance with the requirements of the law. It is
not intended to be exhaustive nor to limit the invention to the
precise form(s) described, but only to enable others skilled in the
art to understand how the invention may be suited for a particular
use or implementation. The possibility of modifications and
variations will be apparent to practitioners skilled in the art. No
limitation is intended by the description of exemplary embodiments
which may have included tolerances, feature dimensions, specific
operating conditions, engineering specifications, or the like, and
which may vary between implementations or with changes to the state
of the art, and no limitation should be implied therefrom.
Applicant has made this disclosure with respect to the current
state of the art, but also contemplates advancements and that
adaptations in the future may take into consideration of those
advancements, namely in accordance with the then current state of
the art. It is intended that the scope of the invention be defined
by the Claims as written and equivalents as applicable. Reference
to a claim element in the singular is not intended to mean "one and
only one" unless explicitly so stated. Moreover, no element,
component, nor method or process step in this disclosure is
intended to be dedicated to the public regardless of whether the
element, component, or step is explicitly recited in the Claims. No
claim element herein is to be construed under the provisions of 35
U.S.C. Sec. 112, sixth paragraph, unless the element is expressly
recited using the phrase "means for . . . " and no method or
process step herein is to be construed under those provisions
unless the step, or steps, are expressly recited using the phrase
"comprising the step(s) of . . . ."
* * * * *